This invention relates to electron guns (sources) for use for instance in electron beam lithography, and to the cathodes (electron emitters) of such guns.
Electron beam columns are well known for use, for instance, in electron beam lithography for imaging a pattern onto a substrate typically coated with a resist sensitive to electron beams. Subsequent development of the exposed resist defines a pattern in the resist which later can be used as a pattern for etching or other processes. Electron beam columns are also used in electron microscopy for imaging surfaces and thin samples. Conventional electron beam columns for electron microscopy and lithography are well known and typically include an electron gun, including an electron emitter, that produces an electron beam. The beam from the gun may be used to produce a scanning probe, or may be used to illuminate a sample or an aperture using a series of electron beam lenses, which are magnetic or electrostatic lenses.
A well-known variant is called a microcolumn which is a very short and small diameter electron beam column typically used in an array of such columns; See xe2x80x9cElectron beam technologyxe2x80x94SEM to microcolumnxe2x80x9d by T. H. P. Chang et al., Microelectronics Engineering, 32 (1996) p. 113-130. See also U.S. Pat. No. 5,122,663 to T. H. P. Chang et al. issued Jun. 16, 1992, also describing microcolumns. These documents are incorporated herein by reference.
Both conventional electron beam columns and microcolumns include a source of electrons. In one version this source is a conventional Schottky emission gun or a field emission gun (generally referred to as electron guns) which typically includes an emitter (cathode) and the triode region surrounding the emitter (see FIG. 1) downstream of which, with respect to the direction of the electron beam, is an electrostatic pre-accelerator lens that focuses and accelerates the electron beam to its final energy. As described above, this gun optics is followed by a series of lenses which refocuses and images the source aperture or sample onto the target.
It has generally been difficult in the prior art to obtain very high beam currents using high brightness electron sources. Although the brightness of the cathode (the electron emitter) is high in such sources, the angular intensity of the electron beam emerging from the emitter region is limited by the properties of the cathode itself. This means that a rather high aperture angle must be used in the electron gun optics. (Optics here refers to structures for handling and manipulation of electron beams and not to conventional light optics.) This makes spherical and chromatic aberration in the gun lens a major factor limiting the beam current that may be obtained in a small spot (spot here refers to the diameter of a cross-section of the beam).
Optimization of such electron guns is described in xe2x80x9cSome general considerations concerning the optics of the field emission illumination system,xe2x80x9d L. H. Veneklasen, Optik, 36 (1972)p. 410-433. This document shows that sources with not only high brightness but also high angular intensity are needed for optimal high current performance.
Modern electron beam lithography systems for use, for instance, in the semiconductor field (mask making or direct writing of integrated circuit patterns on a semiconductor wafer) favor a shaped beam (other than Gaussian in cross-section) so that more than one pixel is exposed simultaneously. Pixel refers to a picture element in the exposed image. Optimum formation of very small high current density shaped beams uses shadow projection optics as disclosed in, for instance, U.S. patent application Ser. No. 09/058,258 filed Apr. 10, 1998 entitled xe2x80x9cShaped Shadow Projection for Electron Beam Columnxe2x80x9d, Lee H. Veneklasen et al., incorporated herein by reference in its entirety. In such optics, the beam shape is a projection shadow of one or more shaping apertures. It has been shown that the distortion of such shadow projection shapes depends upon the spherical aberration of the electron gun and objective lens. This kind of optics requires a high brightness cathode for good feature edge resolution in the projected image and also requires a high angular intensity cathode to minimize gun aberrations. Thus high current shadow projection optics as well as Gaussian optics (referring to a rounded somewhat diffused spot) benefit from a high angular intensity source.
FIG. 1 shows in a side cross sectional view a prior art high brightness electron source and triode region 10 which is typically part of an electron beam column and also referred to generally as an electron gun. The remainder of the electron beam column is not shown. This is exemplary of a field emission or Schottky emission gun. Details of such a device are shown in L. Swanson and G. Schwind, xe2x80x9cA Review of the ZRO/W Schottky cathodexe2x80x9d, Handbook of Charged Particle Optics editor Jon Orloff, CRC Press LLC, New York, (1997) incorporated herein by reference. The depicted rays 26 in FIG. 1 show the limits 29 (envelope) of the useful high brightness electron beam and those electrons 30 passing through the extractor aperture 28. The angular intensity is the total current in beam 30 divided by the solid angle into which the cathode 14 is emitting. The remainder of the xe2x80x9cshankxe2x80x9d emission 20 is thermionic and contributes to the total emission current but not to the useful beam 30 current and hence is effectively wasted, since there it impinges on the outside of the extractor electrode structure 24 and is not part of the final beam 30. The cathode 14 is typically an oriented single crystal tungsten structure with a sharp point (approximately 1 micrometer radius) and mounted on a hair pin filament (not shown).
This assembly is surrounded by the negatively biased suppressor electrode 16 which is typically a conductive structure that prevents thermionically emitted electrons from leaving the cathode 14 anywhere but near its tip. The pointed tip of cathode 14 protrudes slightly from the suppressor electrode 16 and faces the extractor electrode (anode) 24 Which defines small diameter hole 29. The extractor electrode 24 is biased positively with respect to the cathode 14 and defines an aperture 28 below the upper hole 29 to shape the final beam 30 entering the downstream gun lens (not shown). It is the combination of the electric field and temperature that causes emission from low work function facets of the cathode 14.
There have been prior attempts to improve the performance of such Schottky and field emission guns. One attempt is to reduce the aberrations of the electrostatic gun lens. This is difficult using standard electrostatic lenses whose size and focal length are limited by the need for high voltage stand-off distances (the lens voltages are typically extremely high in the thousands or tens of thousands of volts).
It is also possible to use a magnetic lens near the cathode. For an example see xe2x80x9cA New Design of a Field Emission Electron Gun with a Magnetic Lensxe2x80x9d Delong et al., Optik, 81:3 (1989), pp. 103-108. This discloses that the cathode is deeply immersed in the magnetic field of a miniature magnetic lens of low chromatic and spherical aberration. This leads to an appreciable increase in the operating solid angle of emission compared with other designs. The magnetic pole pieces are built into the gun to provide a preaccelerator lens with very low aberrations. This preaccelerator lens uses coils to provide the magnetic lens, and uses two iron pole pieces to form the focusing field. However, this approach disadvantageously requires major modifications of the gun design and geometry and so is not suitable for installation in a conventional electrostatic electron gun as in FIG. 1.
Another approach is to change the electron gun operating conditions so as to increase the angular intensity of the electrons leaving the cathode surface, thus allowing more electrons into the usual acceptance area of the gun lens. This requires increasing the current density and/or increasing the radius of the approximately spherical or faceted cathode tip so that more electrons are emitted into a particular solid angle. The result is a higher extraction voltage and a larger total emission current. Total emission current, the field of the surface and the voltage available for emission are all limited by other considerations (for example, gun design and material). Thus it is not easy to improve the angular intensity of high brightness electron emitters much beyond what they currently provide (one milliamp per solid radian) without adverse side effects. In particular the energy spread increases and it becomes difficult to maintain the cathode shape, emission stability and noise necessary for precision lithography. Hence this approach is not very promising.
In accordance with this invention, performance of Schottky and field emission electron guns whose performance is limited by aberrations is improved. This improvement collimates the electron beam that leaves the emitter (cathode), thus condensing the electron emission from the same area on the cathode into a narrower angle, before it enters the gun lens. A method and structure for such precollimation disclosed here allows standard cathodes to be used with standard electrostatic gun optics, so major electron gun redesign is not required.
Hence a novel modification is made to what is otherwise a standard electron gun cathode assembly in either a conventional electron gun or a microcolumn. In accordance with the invention, the conventional suppressor electrode enclosing the cathode (emitter) is modified to include a small ring shaped permanent magnet surrounding the hole through which the cathode tip protrudes. The tail of the axial magnetic field from this permanent magnet creates a short focal length immersion magnetic lens immediately downstream of the cathode tip. This magnetic field collimates the electron beam before it enters the downstream electrostatic gun lens, thus increasing the effective angular intensity of the electron gun. The aberrations of this collimating lens are very low so its useful brightness is not reduced. However, the influence of gun lens aberrations is reduced because advantageously a smaller aperture angle may be used in the electrostatic preaccelerator gun lens to obtain higher beam current or smaller source image.